With the exhaustion of many sources of high quality petroleum resources, the refining industry has turned to the recovery and refining of less desirable petroleum stocks and to the further refining of residues from refining operations of traditional or higher quality petroleum stocks. The refining of low quality stock or the heavy residues from the refining of high quality stocks present problems for the refining industry with respect to the capability of the refining process to handle more complex hydrocarbons of higher molecular weight as well as increased carbon residue, organometallic, nitrogen and sulfur contaminants.
A traditional refining technique has been the use of a fluidized bed catalytic cracker for refining of petroleum stocks. Despite the agitation and inherent abrasions of catalyst in a fluidized bed reactor, the cracking operation which occurs in petroleum refining leaves the particulate catalyst in an inactivated state due to coke buildup on the surface of the catalyst. In order to perform a relatively steady state operation in the fluidized catalytic cracker, it is necessary to remove coked catalyst from the reactor on a continuous basis and to regenerate such catalyst. The regenerated catalyst is then returned to the fluidized catalytic cracker reactor without shut down of the latter.
Regeneration is usually performed in a fluidized bed with an upflowing oxidant gas at elevated temperatures in the regenerator. In this manner, coke on the catalyst is burned and removed as carbon monoxide, carbon dioxide and water. This exothermic combustion provides heat which is absorbed by the regenerator catalyst, and the heated catalyst is returned to the fluidized bed catalytic cracker reactor wherein the regeneration heat is utilized in the endothermic cracking process.
As heavier and heavier petroleum stocks are refined in this manner, additional coking of the catalyst occurs, and additional contamination of the catalyst with nitrogen and sulfur constituents is experienced. The removal of such coke requires a given through-put of oxidant gas. As the coke on catalyst value has gone up with the refining of such heavy residual petroleum stocks, the necessity for more gas through-put in the regenerator has occurred, and this requirement has been the limiting factor on the amount of heavy residual petroleum stock which may be processed in the fluidized catalytic cracker reaction zone. Additionally, as coke on catalyst goes up, more coke burning occurs in regeneration and added heat releases become a limitation on residuum processing.
The traditional oxidant gas used in a regenerator is air. In order to meet the through-put requirements of regenerators experiencing elevated levels of coke on catalyst, the industry has utilized oxygen-enriched air mixtures in order to complete the combustion requirements necessary for removal of the coke from the catalyst [without exceeding velocity limitations]. However, this results in a regenerator temperature increase. McGovern, et al. in U.S. Pat. No. 4,370,222 teaches that oxygen enriched air coupled with heat removal from the catalyst by steam coils or catalyst coolers can achieve increased coke burning capacity within temperature and velocity constraints.
Miguerian, et al. in U.S. Pat. No. 4,300,997 have taught a hot regeneration technique using special catalyst in the fluidized catalytic cracker that promotes the burning of carbon monoxide in carbon dioxide with attendant higher temperatures and velocities in the regenerator and increased heat release. The special catalyst sorbs at least some of the sulfur oxides, but the nitrogen oxides remain untreated.
It is known to utilize an oxygen-containing gas which is diluted with various inert gases. In U.S. Pat. No. 4,146,463, a process is set forth wherein an oxygen-containing gas, such as air is diluted with moderators, such as carbon dioxide, nitrogen or regenerator recycle gas as the oxidant for the coked catalyst. Sulfur oxides and carbon monoxide from other portions of the refinery are also introduced into the regenerator.
In U.S. Pat. No. 4,118,339, a process is disclosed wherein the effluent gas from a fluidized catalytic cracker regenerator is controlled by the introduction of a noble metal oxidation promoter-containing solvent into the regeneration zone. The promoter catalyzes essentially complete combustion of the regenerator carbon monoxide and results in operation at higher temperatures with zeolite thermally stable catalysts and avoids the carbon monoxide pollutant problem encountered with previous regeneration techniques. The increased regeneration gas, heat release and regeneration temperature, however, intensify these limitations with heavy residuum feeds.
U.S. Pat. No. 4,274,942 discloses a process for the regeneration of fluidized catalytic cracker catalyt wherein control of the sulfur oxide emissions is performed by sensing the output from the regeneration zone and pretreating the coked catalyst with steam before regeneration is performed.
In the conventional regeneration of catalyst, air or oxygen-enriched air results in a large amount of nitrogen being passed through the regenerating catalyst with no beneficial effect. Such effluent gases generally contain nitrogen, carbon dioxide, carbon monoxide, oxygen, hydrogen sulfide, sulfur oxides and nitrogen oxides. The high nitrogen content of the effluent gas renders the recovery of the carbon dioxide and the conversion of carbon monoxide to hydrogen impractical and uneconomical. In addition, the sulfur and nitrogen oxides and the carbon monoxide constitute a pollution problem. This problem is heightened by the processing of heavy residuum which contains high levels of nitrogen and sulfur constituents.
The use of mixtures of essentially pure oxygen diluted with flue gas or other inert gases, such as CO.sub.2, for the fluidization/combustion gas mixture in an FCC regenerator is taught by Pratt, et al. in U.S. Pat. No. 4,304,659. Since flue gas is typically 80-90% nitrogen when air is the combustion gas, dilution of pure oxygen with flue gas has nearly the same effect on the regenerator heat balance and gas velocity as oxygen enrichment of air.
The use of a mixture of essentially pure oxygen and CO.sub.2 as the FCC regenerator combustion gas however, has a significant effect on the regenerator heat balance and gas velocity. The benefits of O.sub.2 /CO.sub.2 combustion gas mixtures are taught by Rowe in U.S. Pat. No. 4,388,218. Rowe also recognizes that FCC flue gas containing carbon monoxide can be processed to obtain a CO enriched chemical feedstock. However, no process scheme is proposed to recover CO.sub.2 from the flue gas for recycling to the regenerator, or CO for chemical feedstock, nor is elimination of flue gas sulfur emissions contemplated.
Different dry and wet flue gas scrubbing technologies are available for eliminating more than 90% of the sulfur oxide emissions from the FCC flue gas. Reeder, et al. teaches in U.S. Pat. No. 3,970,740 that catalyst fines and acid gases can be removed by injection of an aqueous scrubbing mixture in a defined pH range controlled by addition of NaOH or other caustic material. This process is practical when scrubbing flue gas containing predominantly nitrogen, but the NaOH consumption is significantly increased when the flue gas is predominantly CO.sub.2. A major water treating problem also results.
Kosseim, et al. in U.S. Pat. No. 3,201,752 teach a process for selective absorption of sulfur oxides from flue gas streams containing 10-15% CO.sub.2 and predominantly nitrogen. Again, flue gas containing predominantly carbon dioxide would result in significant CO.sub.2 coabsorption and make this process unattractive.
Schorfheide teaches the use of mixtures of O.sub.2 and CO.sub.2 to regenerate catalytic reformer noble metal catalyst in U.S. Pat. No. 4,354,925. The benefit of increased heat removal allows regeneration with higher oxygen concentration resulting in significantly increased combustion rates. An integrated continuous recycle process is not proposed for this cyclic, non-steady state regeneration operation.
Additional patents of interest include U.S. Pat. Nos. 2,322,075, 3,838,036, 3,844,973, 4,036,740, 4,176,084, 4,206,038 and 4,300,997.
The prior art has recognized the use of oxygen and carbon dioxide mixtures as combustion gas in FCC catalyst regenerators, as well as in catalytic reformer noble metal catalyst regeneration, to burn coke from the spent catalyst and thereby regenerate it for further use. No where in the prior art is an integrating processing scheme taught for the regeneration of catalyst with a mixture of pure oxygen diluted with carbon dioxide, the recovery and recycle of CO.sub.2 from effluent flue gas to the regenerator, the recovery and processing of the net CO and CO.sub.2 combustion products to saleable products, and the essentially complete elimination of atmospheric emissions by recovery of concentrated SOX, NOX or H.sub.2 S streams for further processing.
While the increased coke burning capacity benefits of using O.sub.2 and CO.sub.2 mixtures for FCC catalyst regeneration have been recognized in the prior art, these mixtures are not used because the cost of oxygen is so high. However, the integrated process of this invention achieves the FCC coke burning capacity benefits while producing hydrogen and carbon dioxide products for sale and eliminating sulfur and nitrogen oxide atmospheric emissions completely, resulting in an economically attractive way to process high coke yielding feedstocks in an FCC unit without the addition of internal heat removal system to cool the catalyst.
Other fluidized processing systems wherein hydrocarbonaceous coke is removed from particulate matter by combustion in air are described by U.S. Pat. No. 4,243,514 for the treatment of asphalt residuum in a bed of inert particulate matter. U.S. Pat. No. 2,527,575 for fluid coking of residuum, and U.S. Pat. No. 3,661,543 for gasifying fluidized coke. The use of pure oxygen diluted with carbon dioxide can benefit these processes in the same manner as it benefits the regeneration of fluid catalytic cracking catalyst, but the cost of oxygen and carbon dioxide have up to now prevented commercial applications.